W. W. Watts—On Outcrops. 307 
traced by means of contour lines. The main results gained from 
this method may be thus summed up :— 
1. A bed of rock parallel with the waterway of a valley crops out 
in two parallel lines. If the plane be turned through 180°, the lines 
begin to meet (a) down the valley where the inclination is in the 
same direction as, and greater than, that of the waterway ; in every 
other case (b), they meet up the valley. 
2. Where the strike crosses the waterway obliquely, the sharpest 
change in the outcrop line will be on that side of the valley where 
the acute angle made by the strike with the contour lines is smallest 
in case (a), and greatest in case (0). 
Fic. 1. Fie. 2. 
Fre. 1. Outcrop in valley; strike oblique ; dip with waterway. 
Fie. 2. Ditto $5 3 ; dip against waterway. 
Strike-faults.—Sometimes one is apt to think that the whole 
nomenclature of faults is unsatisfactory, and some of the terms used, 
hade particularly, misleading. But a little careful consideration will 
convince us that it is quite right to refer the inclination of hade to 
a vertical plane and not to one at right angles to the strata; for the 
two forces responsible for the results of faults are gravitation and 
crust-crushing, and these two forces together determine the direction 
of movement. After drawing all the possible cases of strike fault- 
ing it is clear that all normal faults, as ordinarily defined, are due 
to movement under the influence of gravitation where rocks have 
been stretched and cracked, and have moved so as to gain space. 
Reversed faults, on the other hand, are those in which compression 
has occurred and space has been saved. 
The result is that all normal faults, with one exceptional type, 
tend to repeat the outcrop of beds, while reversed faults, with a 
parallel exception, have a tendency to conceal beds. As the excep- 
tions belong to types of frequent occurrence, Figs. 3 and 4 illus- 
